Injection of electric charge carriers into non-conducting materials



July 22, 1969 w. HELFRICH ET AL 3,457,153

INJECTION OF ELECTRIC CHARGE CARRIERS INTO NONCONDUCTTNG MATERIALS Filed Jan. 4, 1966 IN VIN 7'0 Wola n 3 He lf'm'dw William 6. 5c )2 a liar {True/115x:

3,457,153 INJECTION OF ELECTRIC CHARGE CARRIERS INTO NON-CONDUCTING MATERIALS Wolfgang Helfrich, Ottawa, Ontario, and William G. Schneider, Rockcliife, Ontario, Canada, assignors to Canadian Patents and Development Limited, Ottawa, Ontario, Canada, a corporation of Canada Filed Jan. 4, 1966, Ser. No. 518,604 Int. Cl. B01k 1/00 U.S. Cl. 204-131 5 Claims ABSTRACT OF THE DISCLOSURE Injecting electric charge carriers (electrons and holes) into an aromatic, crystalline, electrically non-conducting material such as anthracene by contacting opposite faces of the material with electrolytes containing ions of the materials of opposite polarity and applying a potential across the material to each of the electrolytes.

This invention relates to methods of injecting electric charge carriers (electrons and holes) into materials that are normally highly electrically non-conducting.

While, as indicated below, the invention is not restricted in its broad scope to the injection of charges into any one particular material, it will be convenient to take as a typical material a single crystal of anthracene. Anthracene is comparatively readily available in highly pure form, and has for this reason been adopted as the principal material employed-in the experimental work leading up to the present invention.

Effective injection of electrons into anthracene or similar crystalline materials has not previously been achieved. H. Kallmann and M. Pope reported in J. Chem. Phys, 32, 300 (1960) the injection of holes into anthracene, but only very small currents were achieved.

By means of the technique of the present invention, which is described below, it has now proved possible both to obtain an improved injection of holes into such materials (currents of the order of 100 times greater than those obtained by Kallmann and Pope) and to obtain for the first time a similar injection of electrons.

Such improved facility enables a crystal of anthracene to function as a rectifier of either polarity, depending upon whether it is chosen to inject holes or electrons. Moreover, it has now become possible for the first time to inject holes and electrons simultaneously from opposite faces of the crystal. When this was done it was dis covered that the fiow of doubly-injected current caused the emission of blue light from within the crystal. This light which was observed from the crystal edges (since the electrodes contacting the faces were opaque solutions) consisted of the fluorescence spectrum of anthracene, with the first two peaks on the short-wavelength side suppressed, presumably because of reabsorption of these components by the crystal. It could clearly be seen that the light was generated in the crystal, rather than at an electrode surface, and is believed to be the result of recombination of holes and electrons in the body of the crystal.

According to the invention the improved facility to inject holes and/ or electrons is achieved by employing as the injecting electrode a solution containing ions of the same substance as that of the crystal itself. Thus, when the crystal is anthracene, the electrode must contain anthracene ions.

If the electrode is to be used to inject holes, then a solution of positive anthracene ions is required, which can be prepared, by way of example, by adding aluminum chloride to a solution of anthracene in nitromethane. As

nited States Patent O Patented July 22, 1969 an alternative, sulphuric acid may be used as a reagent which, when anthracene is added, produces and dissolves positive anthracene ions.

If the electrode is to be used to inject electrons, then a solution of negative anthracene ions is required, which can be prepared, by way of example, by interacting metallic sodium with a solution of anthracene in tetrahydrofuran. As alternatives, the tetrahydrofu-ran can be replaced by dimethoxyethane or dimethyl sultoxide, or by dimethyl formamide, and the sodium by other alkali metals.

For rectifier applications where one non-injecting electrode is required, a metal, or an aqueous solution of sodium sulfate, or some other suitable material can conveniently serve as such an electrode.

The broad scope of the invention extends to analogues of anthracene and to other crystalline aromatic compounds, provided that the injecting electrode or electrodes always comprises a solution containing ions of the compound chosen as the crystal, positive or negative as the case may be. Alternatively, solutions may be employed which dissolve some of the crystal material and generate the required ions in situ.

Assemblies of this kind may be employed as light generators.

Apparatus for carrying the invention into practice is illustrated diagrammatically and by Way of example in the single figure of the accompanying drawings.

A thin crystal of anthracene 10 is held clamped between a pair of polystyrene or glass plates 11. Glass tubes 12 are fused or cemented at their ends with epoxy cement both to the plates 11 and the crystal 10, and each tube 12 leads to a bowl 13 closed by a stopper 14. An electric potential is applied to each of the liquid electrodes 15 and 16 by a respective platinum wire 17. If necessary, having regard to the nature of the solution, an inert atmosphere, e.g. nitrogen, may be used in the space above its free surface. The respective solutions will be solutions of anthracene ions, as indicated above, or solutions of ions of whatever other crystalline material is chosen to replace the anthracene crystal.

The crystal will preferably be made comparatively thin (e.g. 5 mm. or less), in order to minimize the distance the carriers must travel, since much of the applied energy is used in bringing about this travel. Apart from such losses, the light is generated at nearly maximum efiiciency, as almost one blue light quantum is gene-rated for every electron injected. (The current being volume-controlled, holes and electrons are injected at equal rates.) It was found that, with voltages of 1000 volts and higher applied to an anthracene crystal by injecting electrodes containing anthracene ions, steady state currents as high as 10* amps. cm. are obtainable without observing saturation effects. This compares with prior hole injection currents of the order of only 10- to l0 amps. cm. when the electrode did not contain the ions of the crystal.

We claim:

1. A method of injecting charged electric carriers into an aromatic, crystalline, electrically non-conducting material comprising contacting a first face of said material with a first liquid electrode containing a first solution of ions of said material, contacting a second face of said material with a second liquid electrode containing a second solution of ions of said material, and applying to said first electrode a first electric potential different from a second potential applied to said second electrode, the ions of said first solution being of the same polarity as said first potential bears in relation to said second potential, and the ions of said second solution being of the same polarity as said second potential bears in relation to said first potential.

2. A method according to claim 1, wherein said material is anthracene.

3. A method according to claim 2, wherein said first potential is negative and said first solution contains negative anthracene ions.

4. A method according to claim 2, wherein said first potential is positive and said first solution contains positive anthracene ions.

5. A method according to claim 1, wherein said ma- 0nd potential is positiveand said second solution contains positive anthracene ions.

References Cited Pope et al.: The Journal of Chemical Physics, vol. 36, No. 9, May 1, 1962, pp. 2486-2492.

HOWARD S. WILLIAMS, Primary Examiner terial is anthracene, said first potential is negative, said 10 SIDNEY KANTER, Assistant EXaminer first solution contains negative anthracene ions, said sec- 

